Glucose fuels life and is a central nutrient signal for growth regulation in a broad range of organisms from E. coli, yeasts to plants and humans. Despite the essential and multifaceted regulatory roles of glucose in gene expression, physiology, metabolism, cell proliferation, growth and development, and human diseases, the molecular and cellular mechanisms of glucose signaling remain elusive in multicellular plants and animals. Our research in the model plant Arabidopsis thaliana has provided compelling molecular, chemical and genetic evidence that hexokinase1 (HXK1) and target-of-rapamycin (TOR) kinase are two evolutionarily conserved master regulators in glucose signaling, which integrate direct glucose sensing and glucose-driven energy signaling to orchestrate transcriptional networks and plant growth in response to environmental cues. Our recent findings uncover two surprising and distinct functions of Arabidopsis HXK1 that mediate glucose signaling without its catalytic activity. In leaves, HXK1 senses excess glucose at low nitrate and acts in the nucleus to modulate transcriptional reprogramming. In nitrate sufficient conditions, HXK1 plays an additional novel function in direct binding and mobilization of the phytohormone auxin to promote cell and organ size and growth. We have also developed new chemical genetic tools to discover a previously unrecognized central role of glucose-TOR signaling in controlling stem/progenitor cell proliferation in meristem activation and postembryonic plant growth. The goal of this research project is to elucidate the molecular mechanisms of glucose signaling controlled by the glucose sensor HXK1 and the energy sensor TOR kinase in Arabidopsis. The proposed experiments are designed to integrate molecular, biochemical, cellular, genetic, and genomic approaches to reach a comprehensive understanding on major branches of glucose signaling mechanisms central to plant growth, including the unconventional role of HXK1 in transcriptional reprogramming, the novel function of HXK1 in auxin binding and mobilization, as well as the transcriptional network modulated by glucose-TOR signaling. The project on uncovering the novel glucose-HXK1 and glucose-TOR signaling mechanisms will establish new paradigms in glucose responses and regulations in plants and animals, and build a new conceptual framework to enhance our understanding of the molecular and cellular mechanisms of glucose signaling from plants to humans. Three Specific Aims are: Aim 1. Elucidate the regulatory mechanism of HXK1 functions as a nuclear glucose sensor. Aim 2. Characterize the novel HXK1 functions in auxin and glucose synergism. Aim 3. Define the novel glucose-TOR signaling network gating the cell cycle entry.